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| United States Patent |
6,246,048
|
|
Ramos
, et al.
|
June 12, 2001
|
Methods and apparatus for mechanically enhancing the sensitivity of
longitudinally loaded fiber optic sensors
Abstract
An optical fiber is provided with a Bragg grating formed along a portion of
its core and a mechanical structure arranged adjacent to the Bragg grating
for amplifying longitudinal strain on the fiber in the vicinity of the
grating. The mechanical structure is designed to convert ambient pressure
into longitudinal strain on the fiber in the vicinity of the grating and
to allow the fiber to pass through the structure so that several pressure
measuring apparatus may be arranged along a single optical fiber. An
intermediate structure is provided between the fiber and the mechanical
structure for minimizing buckling of the fiber. The methods of the
invention include converting pressure into longitudinal strain on an
optical fiber, amplifying the effect of pressure on the longitudinal
strain, measuring pressure by determining the spectral location related to
peaks (or minimums) of light reflected from an optical grating subjected
to longitudinal strain.
| Inventors:
|
Ramos; Rogerio T. (Bethel, CT);
Madhavan; Raghu (Brookfield, CT);
Yamate; Tsutomu (Brookfield, CT);
Balkunas; Stephen C. (Brookfield, CT);
Schroeder; Robert J. (Newtown, CT)
|
| Assignee:
|
Schlumberger Technology Corporation (Ridgefield, CT)
|
| Appl. No.:
|
313603 |
| Filed:
|
May 18, 1999 |
| Current U.S. Class: |
250/227.18; 250/227.21; 385/77 |
| Intern'l Class: |
H01J 005/16 |
| Field of Search: |
250/227.14,227.17,227.18,227.21
356/32,35.5
385/77,85
|
References Cited
U.S. Patent Documents
| 4593969 | Jun., 1986 | Goodman et al. | 350/96.
|
| 4755668 | Jul., 1988 | Davis | 250/227.
|
| 4834493 | May., 1989 | Cahill et al. | 385/77.
|
| 5026984 | Jun., 1991 | Gerdt | 250/227.
|
| 5646401 | Jul., 1997 | Udd | 250/227.
|
| 5841131 | Nov., 1998 | Schroeder et al. | 250/227.
|
| 5844667 | Dec., 1998 | Maron | 356/35.
|
| Foreign Patent Documents |
| 41 14 199 | Nov., 1992 | DE | .
|
| 196 48 403 C1 | Apr., 1998 | DE | .
|
| 198 07 891 A1 | Aug., 1999 | DE | .
|
| 0 538 779 A2 | Oct., 1992 | EP | .
|
| 0 525 717 A1 | Feb., 1993 | EP | .
|
| WO98/31987 | Jul., 1998 | WO | .
|
| WO 99/32911 | Jul., 1999 | WO | .
|
Other References
Xu, M.G. et al. Optical In-Fibre Grating High Pressure Sensor. Electronics
Letters. vol. 29, No. 4, (1993), pp. 398-399.
Xu, M.G. et al. Fibre Grating Pressure Sensor with Enhanced Sensitivity
Using a Glass-Bubble Housing. Electronics Letters. vol. 32, No. 2, (1993),
pp. 128-129.
|
Primary Examiner: Allen; Stephone B.
Attorney, Agent or Firm: Batzer; William B., Gordon; David P.
Claims
What is claimed is:
1. A fiber optic transducer, comprising:
a) a fiber optic having a core having at least one grating formed along at
least one portion of said core;
b) a mechanical structure coupled to said fiber optic core which converts
pressure and/or temperature on said mechanical structure to longitudinal
strain on said fiber optic core at said grating; and
c) an intermediate structure between said fiber optic and said mechanical
structure, wherein
said mechanical structure is adapted to allow said fiber optic to pass
through and exit said mechanical structure, said fiber optic passes
through and exits said mechanical structure, and said intermediate
structure is adapted to minimize buckling of, said fiber optic.
2. A fiber optic transducer according to claim 1, wherein:
said mechanical structure includes a tube covering said fiber optic, said
tube having two ends, a pair of sealing members, each end of said tube
being sealed by one of said sealing members which physically couples said
fiber optic to said tube.
3. A fiber optic transducer according to claim 2, wherein:
said intermediate structure includes a soft filling between said tube and
said fiber optic.
4. A fiber optic transducer according to claim 3, wherein:
said soft filling has a Young's modulus much lower than that of said tube.
5. A fiber optic transducer according to claim 4, wherein:
said soft filling is silicon rubber.
6. A fiber optic transducer according to claim 2, wherein:
said intermediate structure includes a filling rod coupled to said fiber
optic in the vicinity of said grating, and
said mechanical structure includes a pair of rigid rods on either side of
said filling rod.
7. A fiber optic transducer according to claim 6, wherein:
said rigid rods are made of a material having a coefficient of thermal
expansion which compensates for the thermal expansion of said tube so that
longitudinal strain on said fiber optic is only the result of changes in
pressure and not the result of changes in temperature.
8. A fiber optic transducer according to claim 6, wherein:
said rigid rods are made of a material having a coefficient of thermal
expansion which enhances the thermal expansion of said fiber so that
longitudinal strain on said fiber optic is mainly the result of changes in
temperature.
9. A fiber optic transducer according to claim 1, wherein:
said mechanical structure includes a housing having a stepped inner
diameter defining two end cavities and a middle cavity, a diaphragm
covering one of said end cavities, and a rigid rod coupled to said
diaphragm and extending partially into said middle cavity, and
said intermediate structure includes a filling rod coupled to said fiber
optic in the vicinity of said grating, said filling rod located in said
middle cavity adjacent said rigid rod.
10. A fiber optic transducer according to claim 9, wherein:
said mechanical structure includes two diaphragms, one covering each end
cavity, and two rigid rods, each rigid rod being coupled to a respective
diaphragm and partially entering said middle cavity.
11. A fiber optic transducer according to claim 10, wherein:
each of said diaphragms defines a hole through which said fiber optic
passes.
12. A fiber optic transducer according to claim 10, wherein:
each of said end cavities defines a side hole through which said fiber
optic passes.
13. A fiber optic transducer according to claim 12, wherein:
each of said rigid rods defines a side hole through which said fiber optic
passes.
14. A fiber optic transducer according to claim 9, wherein:
said diaphragm covers a first one of said two end cavities, a second one of
said two end cavities being sealed with said fiber optic passing
therethrough.
15. A fiber optic transducer according to claim 14, wherein:
said diaphragm defines a hole through which said fiber optic passes.
16. A fiber optic transducer according to claim 14, wherein:
said first one of said end cavities defines a side hole through which said
fiber optic passes.
17. A fiber optic transducer according to claim 16, wherein:
said rigid rod defines a side hole through which said fiber optic passes.
18. A fiber optic sensing system, comprising:
a) a light source;
b) a spectral analyzer; and
c) a fiber optic transducer including
i) a fiber optic having a core with at least one grating formed along at
least one portion thereof,
ii) pressure and/or temperature responsive means for generating
longitudinal strain on said core at said grating, and
iii) an intermediate structure between said fiber optic and said mechanical
structure, wherein
said light source is arranged to direct light into said core and said
spectral analyzer is arranged to detect light exiting said core,
said pressure and/or temperature responsive means is arranged to allow said
fiber optic to pass through and exit said pressure and/or temperature
responsive means and said fiber optic passes through and exits said
pressure and/or temperature responsive means, and
said intermediate structure is adapted to minimize buckling of said fiber
optic.
19. A fiber optic sensing system according to claim 18, wherein:
said pressure and/or temperature responsive means includes a tube, said
tube having two ends, a pair of sealing members, each end of said tube
being sealed by one of said sealing members which physically couples said
fiber optic to said tube.
20. A fiber optic sensing system according to claim 19, wherein:
said intermediate structure includes a soft filling between said tube and
said fiber optic.
21. A fiber optic sensing system according to claim 20, wherein:
said soft filling has a Young's modulus much lower than that of said tube.
22. A fiber optic sensing system according to claim 21, wherein:
said soft filling is silicon rubber.
23. A fiber optic sensing system according to claim 19, wherein:
said intermediate structure includes a filling rod coupled to said fiber
optic in the vicinity of said grating, and
said pressure and/or temperature responsive means a pair of rigid rods on
either side of said filling rod.
24. A fiber optic sensing system according to claim 23, wherein:
said rigid rods are made of a material having a coefficient of thermal
expansion which compensates for the thermal expansion of said tube so that
longitudinal strain on said fiber optic is substantially the result of
changes in pressure only.
25. A fiber optic sensing system according to claim 23, wherein:
said rigid rods are made of a material having a coefficient of thermal
expansion which enhances the thermal expansion of said fiber so that
longitudinal strain on said fiber optic is substantially the result of
changes in temperature only.
26. A fiber optic sensing system according to claim 18, wherein:
said pressure responsive means includes a substantially cylindrical housing
having a stepped inner diameter defining two end cavities and a middle
cavity, a diaphragm covering one of said end cavities, and a rigid rod
coupled to said diaphragm and extending partially into said middle cavity,
and
said intermediate structure includes a filling rod coupled to said fiber
optic in the vicinity of said grating, said filling rod located in said
middle cavity adjacent said rigid rod.
27. A fiber optic sensing system according to claim 26, wherein:
said pressure responsive means includes two diaphragms, one covering each
end cavity and two rigid rods, each rigid rod being coupled to a
respective diaphragm and partially entering said middle cavity.
28. A fiber optic sensing system according to claim 27, wherein:
each of said diaphragms defines a hole through which said fiber optic
passes.
29. A fiber optic sensing system according to claim 27, wherein:
each of said end cavities defines a side hole through which said fiber
optic passes.
30. A fiber optic sensing system according to claim 29, wherein:
each of said rigid rods defines a side hole through which said fiber optic
passes.
31. A fiber optic sensing system according to claim 26, wherein:
said diaphragm covers a first one of said two end cavities, a second one of
said two end cavities being sealed with said fiber optic passing
therethrough.
32. A fiber optic sensing system according to claim 31, wherein:
said diaphragm defines a hole through which said fiber optic passes.
33. A fiber optic sensing system according to claim 31, wherein:
said first one of said end cavities defines a side hole through which said
fiber optic passes.
34. A fiber optic sensing system according to claim 33, wherein:
said rigid rod defines a side hole through which said fiber optic passes.
35. A fiber optic sensing system, comprising:
a) a light source;
b) a spectral analyzer;
c) a fiber optic having a core with a plurality of spaced apart gratings
formed along at least one portion thereof;
d) a plurality of pressure and/or temperature responsive means for
generating longitudinal strain on said core at respective gratings;
e) a corresponding plurality of intermediate structures, each being
arranged between said fiber optic and one of said plurality of pressure
and/or temperature responsive means, wherein
said light source is arranged to direct light into said core and said
spectral analyzer is arranged to detect light exiting said core,
said pressure and/or temperature responsive means are each adapted to allow
said fiber optic to pass through and exit said pressure and/or temperature
responsive means and said fiber optic passes through and exits said
pressure responsive means, and
said intermediate structure is adapted to minimize buckling of said fiber
optic.
36. A method of measuring pressure, comprising:
a) optically coupling a fiber optic grating transducer to a light source,
the fiber optic grating transducer including a mechanical structure for
converting pressure to longitudinal strain on the grating of the fiber
optic, the mechanical structure being arranged to allow the fiber optic to
pass through and exit the mechanical structure and the fiber optic passing
through and exiting the mechanical structure, and an intermediate
structure between the fiber optic and the mechanical structure, the
intermediate structure being adapted to minimize buckling of the fiber
optic.;
b) directing light from the light source into the core of the fiber optic
grating transducer;
c) optically coupling a spectral analyzer to the fiber optic grating
transducer; and
d) measuring the spectral location related to a spectral peak detected by
the spectral analyzer to determine the pressure ambient to the fiber optic
grating transducer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to fiber optic sensors. More particularly, the
invention relates to methods and apparatus for mechanically enhancing the
sensitivity of longitudinally loaded fiber optic sensors and for
converting pressure or temperature to longitudinal strain on a fiber optic
sensor.
2. State of the Art
Fiber optic sensor technology has developed concurrently with fiber optic
telecommunication technology. The physical aspects of optical fibers which
enable them to act as wave guides for light are affected by environmental
influences such as temperature, pressure, and strain. These aspects of
optical fibers which may be considered a disadvantage to the
telecommunications industry are an important advantage to the fiber optic
sensor industry.
Optical fibers, whether used in telecommunications or as environmental
sensors, generally include a cylindrical core, a concentric cylindrical
cladding surrounding the core, and a concentric cylindrical protective
jacket or buffer surrounding the cladding. The core is made of transparent
glass or plastic having a certain index of refraction. The cladding is
also made of transparent glass or plastic, but having a different,
smaller, index of refraction. The ability of the optical fiber to act as a
bendable waveguide is largely determined by the relative refractive
indices of the core and the cladding.
The refractive index of a transparent medium is the ratio of the velocity
of light in a vacuum to the velocity of light in the medium. As a beam of
light enters a medium, the change in velocity causes the beam to change
direction. More specifically, as a beam of light travels from one medium
into another medium, the beam changes direction at the interface of the
two media. In addition to changing direction at the interface of two
media, a portion of the incident beam is reflected at the interface such
that the energy of the beam travelling through the second medium is
diminished (the sum of the energy of the refracted and reflected beams
must equal the energy of the incident beam). The angles of reflection and
refraction can be predicted using Snell's law if the refractive indices of
both media are known.
By altering the indices of refraction of two adjacent media, the angle of
refraction and the angle of reflection of a beam travelling toward the
interface of the two media can be altered such that the intensity of the
light entering the second medium approaches zero and substantially all of
the light is reflected at the interface. Conversely, for any two
transparent media, there is a critical angle of incidence at their
interface at or below which substantially all of the incident light will
be reflected. This phenomenon, known as total internal reflection, is
applied in choosing the refractive indices of the core and the cladding in
optical fibers so that light may propagate through the core of the fiber
with minimal power loss.
As mentioned above, fiber optic sensors employ the fact that environmental
effects can alter the amplitude, phase, frequency, spectral content, or
polarization of light propagated through an optical fiber. The primary
advantages of fiber optic sensors include their ability to be light
weight, very small, passive, energy efficient, rugged, and immune to
electromagnetic interference. In addition, fiber optic sensors have the
potential for very high sensitivity, large dynamic range, and wide
bandwidth. Further, a certain class of fiber sensors may be distributed or
multiplexed along a length of fiber. They may also be embedded into
materials.
State of the art fiber optic sensors can be classified as either
"extrinsic" or "intrinsic". Extrinsic sensors rely on some other device
being coupled to the fiber optic in order to translate environmental
effects into changes in the properties of the light in the fiber optic.
Intrinsic sensors rely only on the properties of the optical fiber in
order to measure ambient environmental effects. Known fiber optic sensors
include linear position sensors, rotational position sensors, fluid level
sensors, temperature sensors, strain gauges, fiber optic gyroscopes, and
pressure sensors.
One type of fiber optic sensor utilizes intra-core fiber gratings.
Intra-core Bragg gratings are formed in a fiber optic by doping an optical
fiber with material such as germania and then exposing the side of the
fiber to an interference pattern to produce sinusoidal variations in the
refractive index of the core. Two presently known methods of providing the
interference pattern are by holographic imaging and by phase mask grating.
Holographic imaging utilizes two short wavelength (usually 240 nm) laser
beams which are imaged through the side of a fiber core to form the
interference pattern. The bright fringes of the interference pattern cause
the index of refraction of the core to be "modulated" resulting in the
formation of a fiber grating. Similar results are obtained using short
pulses of laser light, writing fiber gratings line by line through the use
of phase masks. By adjusting the fringe spacing of the interference
pattern, the periodic index of refraction can be varied as desired.
It has been demonstrated that an ultrahigh hydrostatic pressure induces
fractional changes in the physical length of a fiber optic and thus
induces a fractional change in the Bragg wavelength of a grating
incorporated in the fiber core. For example, M. G. Xu et al., Optical
In-Fibre Grating High Pressure Sensor, Electron. Lett., Vol. 29, No. 4,
pp. 398-399 (1993), demonstrates how a fiber optic Bragg grating sensor
can be used to measure very high pressure. In particular, the Xu et al.
paper demonstrates a simple in-fiber grating sensor which exhibits a
linear Bragg wavelength shift of 3.04.times.10.sup.-3 nm/MPa. The authors
note that the sensor is also sensitive to changes in temperature. They
note a linear Bragg wavelength shift of 10.45.times.10.sup.-3 nm/.degree.
C. and specifically state that far more compensation for the effects of
temperature is necessary for their sensor to be valuable as a pressure
sensor and that the real advantage of their sensor is only evident at
ultrahigh pressure.
It has been suggested that a mechanical structure be attached to a Bragg
grating sensor in order to enhance its sensitivity to pressure. For
example, M. G. Xu et al., Fibre Grating Pressure Sensor with Enhanced
Sensitivity Using a Glass-Bubble Housing, Electron. Lett., Vol. 32, No. 2,
pp. 128-129 (1993), demonstrates how pressure sensitivity is enhanced by
housing the fiber with Bragg grating in a glass bubble. When the glass
bubble is pressurized, the fractional change in the diameter of the glass
bubble .DELTA.d/d owing to a pressure change .DELTA.P is given by Equation
1 where E is the Youngs modulus of the bubble, .mu. is the Poisson ratio
of the bubble, and t is the wall thickness of the bubble.
##EQU1##
If there is good bonding between the fiber and the glass bubble, the
pressure induced strain on the grating is equal to the fractional change
in the diameter of the glass bubble .DELTA.d/d. The pressure sensitivity,
defined as the fractional change in the Bragg wavelength
.DELTA..lambda..sub.B /.lambda..sub.B is given by Equation 2 where P.sub.e
=0.22 is the effective photoelastic constant for silica.
##EQU2##
The glass bubble increased pressure sensitivity of the Bragg grating by a
factor of four. It would seem, however, that the glass bubble structure
would not be suitable for use in harsh environments.
WO 98/31987 to Maron et al. discloses a multiparameter fiber optic sensor
for use in harsh environments such as in the borehole of an oil well. The
sensor generally includes a fiber optic having three or four spaced apart
Bragg gratings all mounted in a single capillary tube with a diaphragm
bonded to one end of the capillary tube. Various materials are located
between the fiber optic and the capillary tube along the length of the
capillary tube and adjacent the Bragg gratings. The three or four spaced
apart Bragg gratings provide a pressure sensor, an acceleration (or
vibration) sensor, and a temperature sensor. Each of the sensors is
isolated from the other sensors by "rigid elements" located between the
fiber optic and the capillary tube. The pressure sensor is activated by
the diaphragm at the end of the capillary tube which causes material
surrounding the closest Bragg grating to place an axial strain on the
Bragg grating. The acceleration sensor is activated by a free moving mass
which impacts a rigid member adjacent to the next Bragg grating and
axially strains the grating in proportion to the acceleration of the mass.
The temperature sensor(s) are formed by one or two Bragg gratings adjacent
one or two rigid members near the end of the tube opposite the end having
the diaphragm. One of the disadvantages of the multiparamter sensor
described by Maron et al. is that the pressure sensor must be located at
the end of the device with a diaphragm arranged orthogonal to the end of
the fiber optic. This prevents the arrangement of several pressure sensors
along a single fiber optic unless beam splitters are used to branch out
the fiber. As mentioned above, one of the inherent advantages of Bragg
grating fiber optic sensors is that many sensors may be arranged along a
long length of single fiber through the use of wavelength or time division
multiplexing.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide methods and apparatus
for enhancing the sensitivity of fiber optic sensors.
It is also an object of the invention to provide methods and apparatus for
enhancing the sensitivity of longitudinally loaded fiber optic sensors.
It is another object of the invention to provide methods and apparatus for
mechanically enhancing the sensitivity of longitudinally loaded fiber
optic sensors.
It is an additional object of the invention to provide a fiber optic
pressure or temperature sensor which has the advantages of a
longitudinally loaded fiber optic strain sensor.
It is also an object of the invention to provide fiber optic pressure
sensors which may be coupled to each other along a single fiber.
In accord with these objects which will be discussed in detail below, the
methods and apparatus of the present invention include an optical fiber
with a Bragg grating formed along a portion of its core and a mechanical
structure arranged adjacent to the Bragg grating for increasing
longitudinal strain on the fiber in the vicinity of the grating, and an
intermediate structure disposed between the fiber and the mechanical
structure to prevent the fiber from buckling. In particular, the
mechanical structure is designed to convert pressure or temperature
ambient to the mechanical structure into longitudinal strain on the fiber
in the vicinity of the grating.
The methods of the invention include converting the effect of pressure or
temperature on a mechanical structure into longitudinal strain on an
optical fiber, amplifying the effect of the pressure on the longitudinal
strain, while preventing the fiber from buckling, and measuring the
pressure by determining the wavelength of the spectral peak (or minimum)
of light reflected from an optical grating subjected to longitudinal
strain.
According to a first embodiment, the mechanical structure includes a tube
which is placed over the fiber optic with the Bragg grating being located
around the midpoint of the tube, a soft filling material between the tube
and the fiber optic, and two end sealings which physically couple the ends
of the tube to portions of the fiber optic adjacent to the ends of the
tube. When the tube is exposed to an increase in ambient pressure, the
entire tube is compressed, the end sealings are moved closer together and
the fiber optic is longitudinally compressed. The filling material
prevents the fiber from buckling. The compression of the optic results in
a longitudinal compression of the Bragg grating which results in a shift
in the wavelength of the spectral peak reflected from the grating. An
increase in pressure results in a shortening of the wavelength. When the
tube is exposed to an increase in ambient temperature, the entire tube
expands, the end sealings are moved apart and the fiber optic is
longitudinally stretched. This results in a longitudinal expansion of the
Bragg grating which results in a shift in the wavelength of the spectral
peak reflected from the grating. An increase in temperature results in a
lengthening of the wavelength. The structure amplifies the effect of
ambient pressure or temperature on the fiber grating according to an
equation which has several variables including the dimensions, Young's
modulus, and Poisson ratio of the tube and the optical fiber. When used as
a pressure sensor, the effects of temperature on the sensor may be
minimized by using a tube which matches the thermal expansion
characteristics of the fiber optic, i.e. fused silica. When used as a
temperature sensor, the effects of pressure on the sensor may be minimized
by isolating the sensor in a chamber. The sensitivity of the sensor to
temperature can be increased by choosing the material of the tube to have
a high thermal expansion coefficient.
According to a second embodiment, the mechanical structure includes a tube
which is placed over the fiber optic with the Bragg grating being located
inside the tube, a filling rod physically coupled to the portion of the
fiber optic containing the Bragg grating, one or more rigid rods inside
the tube on either side of the filling rod, and two end sealings which
physically couple the ends of the tube to respective rigid rods. When the
tube is exposed to an increase in ambient pressure, the entire tube is
compressed, the end sealings are moved closer together, the rigid rods
move closer together compressing the filling rod, and the fiber optic is
thereby longitudinally compressed. This results in a longitudinal
compression of the Bragg grating which results in a shift in the
wavelength of the spectral peak reflected from the grating. The structure
provided in the second embodiment amplifies the effects of ambient
pressure on the fiber grating according to an equation which has the same
variables as the first embodiment equation and also amplifies the effects
of pressure by an additional factor which is related to the ratio of the
distance between the sealings to the length of the filling material. The
effects of temperature on the sensor of the second embodiment can be
minimized by choosing one or both of the rigid rods to be made of a
material having a thermal expansion coefficient which compensates for the
expansion of the tube and the other rod if only one rod is so selected.
The structure of the second embodiment can also be used as a temperature
sensor if it is isolated from the effects of pressure as described above
with reference to the first embodiment. The materials of the tube and rod
in the temperature sensor are also preferably selected in terms of their
thermal expansion coefficient in order to increase the thermal sensitivity
of the sensor.
According to a third embodiment, a filling rod physically coupled to the
portion of the fiber optic containing the Bragg grating and a pair of
rigid rods on either side of the filling rod are arranged in a housing
which is sealed by a pair of diaphragms. Each diaphragm is coupled to a
respective rigid rod. When the structure is exposed to an increase in
ambient pressure, the diaphragms are deflected, the rigid rods move closer
together compressing the filling rod, and the fiber optic is thereby
longitudinally compressed. This results in a longitudinal compression of
the Bragg grating which results in a shift in the wavelength of the
spectral peak reflected from the grating. The structure provided in the
third embodiment amplifies the effects of ambient pressure on the fiber
grating according to an equation which is related to the length of the
filling material, the Young's modulus of the filling material and the
diaphragm, and the geometry of the diaphragms and the rods. According to
one variant of this embodiment, a hole is provided in each diaphragm and
the fiber optic passes through these holes. According to another variant
of this embodiment, side holes in the rigid rods and the housing allow the
fiber optic to pass through the structure. According to another variant of
this embodiment, the structure is formed with a single diaphragm and a
single rigid rod. The effects of temperature on the sensors of the third
embodiment may be adjusted by choosing materials having thermal expansion
coefficients which compensate for the expansion of the several elements of
the transducer.
The pressure sensors of the invention may be used to measure either static
pressure or dynamic (acoustic) pressure. Additional objects and advantages
of the invention will become apparent to those skilled in the art upon
reference to the detailed description taken in conjunction with the
provided figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a fiber optic sensing system according the
invention;
FIG. 2 is a plot illustrating the spectral content of light reflected from
a single Bragg grating subjected to longitudinal strain;
FIG. 3 is a schematic sectional view of a first embodiment of an apparatus
for mechanically enhancing the sensitivity of a longitudinally loaded
fiber optic sensor;
FIG. 4 is a chematic sectional view of a second embodiment of an apparatus
for mechanically enhancing the sensitivity of a longitudinally loaded
fiber optic sensor;
FIG. 5 is a schematic sectional view of a third embodiment of an apparatus
for mechanically enhancing the sensitivity of a longitudinally loaded
fiber optic sensor; and
FIG. 6 is a schematic sectional view of an alternate third embodiment of an
apparatus for mechanically enhancing the sensitivity of a longitudinally
loaded fiber optic sensor.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, an exemplary fiber optic sensor system 10
according to the invention generally includes a light source 12, a
spectral analyzer 14, a fiber beam splitter 16, an optical fiber 18, a low
back reflection terminator 20, and one or more fiber optic transducers 22
(32, etc.) each having one or more gratings 24, 26 (34, 36, etc.). The
light source 12 may be, e.g. an LED, a tunable laser, a laser diode, or a
broadband erbium doped fiber. It is advantageous to use a source which
will permit the application of multiple gratings of different wavelengths.
The spectral analyzer 14 may be a Fabry-Perot interferometer, an
acoustic-optical device, a Michelson interferometer, a Mach-Zehnder
interferometer, or another type of known device. The back reflection
terminator 20 may be of the type disclosed in U.S. Pat. No. 4,834,493 to
Udd et al. The number of transducers and the number of gratings in each
transducer is not critical so long as there is at least one transducer
with one grating. As will be described in more detail below with reference
to FIGS. 3-6, an important feature of the transducers of the present
invention is that the optical fiber passes through the transducer thereby
enabling the arrangement of a plurality of transducers along a single
optical fiber path.
According to the invention, the fiber optic transducer(s) 22 (32) is (are)
also provided with novel structure which produces enhanced sensitivity and
dynamic range as described in more detail below with reference to FIGS.
3-6. The components of the sensing system 10 are arranged substantially as
shown in FIG. 1. The light source 12 directs a beam through the beam
splitter 16 into the optical fiber 18 such that light enters one end of
the fiber optic pressure transducer 22. A spectral portion of the light is
reflected back by the grating(s) 24, 26 (34, 36) to the beam splitter 16
which directs the reflected beam onto the spectral analyzer 14. The other
output end of the beam splitter 16 may be directed to another set (or
sets) of Bragg gratings and pressure transducers (not shown), and if
desired, multiple beam-splitters can be utilized with one or more sets of
gratings on each fiber. In addition, the sensing system can be operated in
a reflection mode as shown, or in a transmission mode with the spectral
analyzer 14 located where the reflection terminator 20 is presently shown.
Regardless, depending on the number of different gratings provided on the
fiber(s), the spectral analyzer will detect one or more spectral peaks as
shown in FIG. 2. The wavelength of the peaks will change based on the
temperature of and longitudinal strain on the gratings respectively. FIG.
2 illustrates the spectral content of light reflected from a single Bragg
grating in an optical fiber subjected to longitudinal strain.
Turning now to FIG. 3, a first embodiment of a transducer 100 according to
the invention includes a tube 102 which is placed over an optical fiber
104 having a Bragg grating 106 located inside the tube 102. As shown in
FIG. 3, the Bragg grating 106 is located near the longitudinal and radial
midpoint of the tube. However, the Bragg grating may be located
longitudinally and/or radially off-center if desired. Regardless, a soft
filling material 108 is placed between the tube 102 and the fiber optic
104, and two end sealings 110, 112 are provided which physically couple
the ends of the tube 102 to portions of the fiber optic 104 adjacent to
the ends of the tube 102. When the tube 102 and end sealings 110, 112 are
exposed to an increase in ambient pressure, the entire tube 102 is
compressed, the end sealings 110, 112 are moved closer together, and the
fiber optic 104 is longitudinally compressed. This results in a
longitudinal compression of the Bragg grating 106 which results in a shift
in the wavelength of the spectral peak reflected from the grating. The
filling material 108 prevents the fiber from buckling. The filling
material is preferably a material having a low Young's modulus as compared
to the tube and the optical fiber, e.g. silicon rubber. The structure of
the tube and end sealings amplifies the effects of ambient pressure on the
fiber grating according to Equation 3, below, where .alpha..sub.1 is the
amplification factor as compared to a bare optical fiber sensor, Y.sub.of
is the Young's modulus of the optical fiber, d.sub.o is the outside
diameter of the tube, y.sub.t is the Young's modulus of the tube, d.sub.i
is the inside diameter of the tube, Y.sub.f is the length of the tube,
v.sub.t is the Poisson ratio of the tube, Y.sub.f is the Young's modulus
of the filling material, d.sub.of is the diameter of the optical fiber,
and v.sub.of is the Poisson ratio of the optical fiber.
##EQU3##
The effects of temperature on the sensor 100 of the first embodiment may be
minimized by using a tube 102 which matches the thermal expansion
characteristics of the optical fiber 104, for example, a fused silica
tube. As mentioned above, the sensor 100 may also be used as a temperature
sensor by containing it in a pressure vessel which prevents pressure
outside the vessel from acting on the sensor but which conducts heat to
the sensor. The sensitivity of the sensor to temperature can be improved
by choosing the material of the tube to have a high thermal expansion
coefficient; i.e., higher than silica.
FIG. 4 shows a second embodiment of a sensor 200 according to the
invention. The sensor 200 includes a tube 202 covering an optical fiber
204 having a Bragg grating 206 located inside the tube 202. A filling rod
209 is physically coupled to the portion of the fiber optic 204 containing
the Bragg grating 206. A pair of rigid rods 211, 213 are located inside
the tube 202 on either side of the filling rod 209. Two end sealings 210,
212 physically couple the ends of the tube 202 to respective rigid rods
211, 213. When the sensor 200 is exposed to an increase in ambient
pressure, the entire tube 202 is compressed, the end sealings 210, 212 are
moved closer together, the rigid rods 211, 213 move closer together
compressing the filling rod 209, and the fiber optic 204 is thereby
longitudinally compressed in the region of the Bragg grating 206. This
results in a longitudinal compression of the Bragg grating 206 which
results in a shift in the wavelength of the spectral peak reflected from
the grating. The structure provided in this second embodiment amplifies
the effects of ambient pressure on the fiber grating according to two
factors, the factor .alpha..sub.1 shown in Equation 3, above, and a second
factor which is defined by Equation 4, below, where .alpha..sub.2 is the
final amplification factor, l.sub.t is the length of the tube 202 and
l.sub.f is the length of the filling rod 209.
##EQU4##
The effects of temperature on the sensor 200 of the second embodiment may
be minimized by choosing one or both of the rigid rods 211, 213 to be made
of a material having a thermal expansion coefficient which compensates for
the thermal expansion of the tube 202 (and which compensates for the
thermal expansion of the other rod if only one rod is so selected). As
mentioned above, the sensor 200 may also be used as a temperature sensor
by containing it in a pressure vessel which prevents pressure outside the
vessel from acting on the sensor but which conducts heat to the sensor.
Likewise, the materials of the tube and rod can be selected in terms of
their thermal expansion coefficients in order to increase the thermal
sensitivty of the sensor.
A third embodiment of the invention is shown in FIG. 5. The sensor 300
shown in FIG. 5 includes a substantially cylindrical housing 302 having a
stepped inner diameter which defines two end cavities 301, 303 and a
middle cavity 305 which has a smaller diameter than the end cavities. An
optical fiber 304 extends through the housing 302 and is provided with a
Bragg grating 306 which is located within the middle cavity 305 of the
housing 302. A filling rod 309 is physically coupled to the portion of the
fiber optic 304 containing the Bragg grating 306. A pair of rigid rods
311, 313 are arranged on either side of the filling rod 309. Respective
rigid rods 311, 313 extend entirely through respective end cavities 301,
303 and partially into the middle cavity 305. The end cavities 301, 303
are sealed by respective diaphragms 310, 312, each of which is provided
with a central hole 310a, 312a through which the optical fiber 304 passes.
The annuli (not shown) between the fiber 304 and the holes 310a, 312aare
sealed to maintain isolated pressure in the cavities 301, 303. Each
diaphragm 310, 312 is coupled to a respective rigid rod 311, 313. When the
structure 300 is exposed to an increase in ambient pressure, the
diaphragms 310, 312 are compressed, the rigid rods 311, 313 move closer
together compressing the filling rod 309, and the optical fiber 304 is
thereby longitudinally compressed in the vicinity of the Bragg grating
306. This results in a longitudinal compression of the Bragg grating 306
which results in a shift in the wavelength of the spectral peak reflected
from the grating. The structure 300 provided in the third embodiment
amplifies the effects of ambient pressure on the fiber grating 306
according to an equation which is related to the length of the filling
material, the Young's modulus of the filling material and diaphragms, and
the geometry of the diaphragms and the rods. The effects of temperature on
the sensor of the third embodiment can be adjusted by choosing materials
of certain transducer elements (e.g., rods) according to their thermal
expansion coefficients in order to compensate for the thermal expansion of
the diaphragms and housing.
A variant of the third embodiment is illustrated in FIG. 6. The sensor
structure 400 shown in FIG. 6 includes a substantially cylindrical housing
402 having a stepped inner diameter which defines two end cavities 401,
403 and a middle cavity 405. The end cavity 401 has a small diameter and
is only large enough to allow an optical fiber 404 to pass therethrough.
The cavity 405 is similar in size to the cavity 305 described above an is
dimensioned to receive the fiber 406 surrounded by a filling rod 409 which
is physically coupled to the portion of the fiber optic 404 containing the
Bragg grating 406. The cavity 403 is relatively large and comparable in
size to the cavities 301, 303 described above. The cavity 403 is covered
by a diaphragm 412 and a hollow rigid rod 413 is arranged extending
entirely through cavity 403 and partially into the middle cavity 405,
abutting the filling rod 409. According to this embodiment, the rigid rod
413 is provided with a side hole 413a adjacent to the cavity 403 and the
housing 402 is provided with a side hole 402aadjacent to the cavity 403.
The optical fiber 404 passes through the cavity 401, through the filling
rod 409 in the cavity 405, through the hollow rigid rod 413, through the
hole 413ain the rigid rod and out of the housing 402 through the side hole
402a. This embodiment obviates the need to seal an annulus between the
fiber and the diaphragm. It will be appreciated that this embodiment
differs from the embodiment shown in FIG. 5 in two ways: the elimination
of holes in diaphragms, and the elimination of one of the two diaphragms.
Those skilled in the art will further appreciate that the embodiments of
FIGS. 5 and 6 may be varied in several ways. For example, the embodiment
of FIG. 5 may be made with only one diaphragm, but without the side hole
passage shown in FIG. 6. Further, the embodiment of FIG. 5 may be made
with two diaphragms and with two side hole passages for the fiber as
suggested by FIG. 6.
The structure shown in FIG. 6 functions in a manner similar to the
structure shown in FIG. 5 and the effects of temperature on the sensor 400
can be minimized in the same manner as described with reference to the
structure 300.
There have been described and illustrated herein several embodiments of
methods and apparatus for measuring pressure with longitudinally loaded
fiber optic sensors. While particular embodiments of the invention have
been described, it is not intended that the invention be limited thereto,
as it is intended that the invention be as broad in scope as the art will
allow and that the specification be read likewise. Thus, it will be
appreciated that various aspects of different embodiments can be utilized
in conjunction with other embodiments of the invention. Also, while
processing to determine pressure by locating spectral peaks was indicated,
it will be appreciated that peak-related spectral locations could be
utilized (e.g., centroids of peaks) instead of actual peaks. Further,
while some components have been described as being circular in cross
section, it will be understood that the tubes and rods could have
non-circular cross sections. It will therefore be appreciated by those
skilled in the art that yet other modifications could be made to the
provided invention without deviating from its spirit and scope as so
claimed.
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